Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
It has been recently proposed to transport natural gas at temperatures above -112.degree. C. (-170.degree. F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. For most natural gas compositions, the pressure of the natural gas at temperatures above -112.degree. C. will be between about 1,380 kPa (200 psia) and about 4,480 kPa (650 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about -162.degree. C. (-260.degree. F.).
In co-pending United States patent application Ser. No. 09/464987 by J. R. Rigby, a process is disclosed for unloading PLNG from ship containers by pressuring out the PLNG with gas, leaving the tanks PLNG-empty but full of pressurized, methane-rich gas. At the end of the PLNG unloading method, all but the last container or group of containers are at low pressure, preferably between about 690 kPa (100 psia) and 1,380 kPa (200 psia), while the last container is at slightly above the original PLNG's bubble point pressure. Having the lower pressure vapor in the containers for the return trip or voyage substantially reduces the mass of methane left in the containers compared to having high-pressure gas contained therein. Depending upon the pressure, temperature, and composition of the PLNG, leaving high pressure vapor in all the containers could constitute from about 10 to 20 percent of the mass of the cargo in the containers before PLNG removal.
During PLNG loading, the methane-rich vapor in the containers is displaced by the entering liquid. It is desirable to liquefy at least part of the methane-rich vapor displaced from containers during PLNG loading. The vapor liquefaction is preferably integrated with the liquefaction process used to manufacture the PLNG being loaded into the containers. During the loading of a multiplicity of gas-filled containers, the flow rate of vapor leaving the containers can vary substantially between the beginning and end of the loading method. To maintain operational stability of the liquefaction plant, it is desirable that the vapor return flow rate be a relatively constant percentage of the plant feed rate. A need exists for a PLNG loading method that provides this type of vapor return flow rate.